Connect user data from form with user who clicked link in autoresponse mail?!

Hello there,

the title of the question may be confusing, I don’t even know if this is possible because I’m still learning all of this, but I wanted to ask if someone has a clue how to approach the following:

What I have so far:
I have a form on a page where the user has to put in some contact information and then gets an automated response email. In this email there is some text and a link to another site where he has to agree to some terms (check checkboxes) before a download button is enabled that lets him download some pdf.

My question now is:
Is there a way to get some information (like email adress) from the person who clicked the link in the autoresponse mail, so I actually know which person filled out the terms of agreement/downloaded the file?
Or maybe something else that gives me a connection between “user who filled out form” (whose data is saved into database after clicking “send”) and “user who agreed to terms” (who clicked the link in the autoresponse mail)?
Since the email is in between this two processes I don’t have any idea if this is possible and how.

I do have some knowledge about plain database/sql stuff but I’m still starting to learn jQuery, PHP, AJAX and so on and have no indepth knowledge. So maybe someone can just tell me if this is even possible an if so, how I should approach it. That would be awesome =)

Thanks a lot!